1) Field of the Invention
This invention pertains to the field of communications and, more particularly, a method and apparatus for processing and modulating digital signals.
2) Background of the Related Art
Typically, in digital communication systems, a digital bit-stream is modulated onto one or more carrier signals for transmission over a communication channel. In some applications, such as certain radiofrequency (RF) communication systems, the modulated signal is passed through a high power amplifier (HPA) before transmission. At the receiver, the signal is filtered, demodulated, and detected as a digital bit-stream.
In many digital communications systems, power efficiency is of great importance. Examples of such systems include certain satellite communications, cellular telephone, mobile and portable radio systems. In such systems, improved power efficiency typically is achieved by operating the HPA nonlinearly at some level of amplitude limiting or saturation.
Maintaining data integrity is another important aspect for digital communication systems. Conventional quadra-phase modulation techniques such as quadrature phase shift keying (QPSK) are widely used in digital communication systems because of, among other things, the desirable error rate performance of systems employing these techniques. In accordance with these techniques, a digital bit-stream is first divided into two bit-streams; an in-phase (I) bit-stream and a quadrature-phase shifted (Q) bit-stream. Carrier signals (typically sine waves) for the I and Q bit-streams are also generated. The I and Q carrier signals are generally synchronous and frequency coherent, but shifted 90 degrees in phase with respect to each other. The I and Q bit-streams modulate the phase of the I and Q sinusoidal carrier signals, respectively. The modulated I and Q carrier signals are then combined to form a composite QPSK signal.
Conventional quadra-phase modulation techniques suffer from certain drawbacks. For example, composite QPSK signals have spectral waveforms which include a relatively large amount of energy in spectral sidelobes, which is undesirable. This undesired spectral sidelobe energy can cause interchannel and/or out-of-band interference resulting in degraded bit error rate performance in a communication system.
Post-modulation filtering has been used to reduce this undesired spectral sidelobe energy. Such filtering is generally impractical, however, after the high power amplifier (HPA). Thus, instead, the QPSK modulated signal is usually filtered before being passed through the HPA.
This type of preamplification filtering of the QPSK modulated signal has its drawbacks. For example, after filtering, the modulated signal can exhibit substantial variation in the amplitude envelope. Thus, when the signal is passed through an amplitude limited or saturated HPA, the signal's spectrum can exhibit substantial sidelobe regrowth or spreading due to amplitude modulation (AM) to phase modulation (PM) conversion.
Modulation techniques have been developed in an attempt to address these problems, particularly that of spectral regrowth. Among these techniques are minimum shift keying (MSK), Gaussian Filtered MSK (GMSK), quadrature overlapped raised cosine (QORC) modulation, and superposed quadrature amplitude modulation (SQAM). These techniques generally incorporate premodulation baseband filtering of the digital bit-stream to reduce spectral sidelobe energy in the modulated signal by smoothing phase transitions between adjacent bit intervals.
Other and further details regarding these and similar previous modulation techniques can be found, for example, in M. Austin, et. al., "QPSK, Staggered QPSK, and MSK--A Comparative Evaluation," IEEE Transactions on Communications, Vol. COM-31, No. 2, pp. 171-182 (1983); M. Austin and M. Chang, "Quadrature Overlapped Raised-Cosine Modulation," IEEE Transactions on Communications, Vol. COM-29, No. 3, pp. 237-249 (1981); J. Soo and K. Feher, "SQAM: A New Superposed QAM Modem Technique," IEEE Transactions on Communications, Vol. COM-33, No. 3, pp. 296-300 (1985); S. Kato and K. Feher, "XPSK: A New Cross-Correlated Phase-Shift Keying Modulation Technique," IEEE Transactions on Communications, Vol. COM-31, No. 5, pp. 701-707 (1983); and K. Murata and K. Hirode, "GMSK Modulation for Digital Mobile Radio Telephony," IEEE Transactions on Communications, Vol. COM-29, No. 7, pp. 1044-1050 (1981).
Accordingly, it would be advantageous to provide a technique for modulating digital signals which produces spectrally efficient waveforms having reduced spectral sidelobe energy, and exhibiting reduced spectral regrowth when passed through a nonlinear, amplitude limited channel, such as a saturated or hard-limited HPA.
It would further be advantageous to provide such a technique well adapted to the transmission of spread spectrum signals.